1. Field of the Invention
The present invention relates to an apparatus and a method for fractionating gypsum slurry and a method of producing gypsum board, and more specifically, to such an apparatus and a method for fractionating the gypsum slurry from a mixer for mixing calcined gypsum and water, and a method of producing the gypsum board with use of the apparatus for fractionating the gypsum slurry.
2. Related Art
Gypsum boards having a gypsum core covered with sheets of paper for gypsum board liner are practically and widely in use for an architectural interior finish material from viewpoints of its advantageous fire-resisting or fire-protecting ability, sound insulation performance, workability, cost performance and so on. In general, a process of producing such a gypsum board comprises a mixing step of admixing a quantity of water and foam (foam for reducing the weight of gypsum board core) with ingredients for the gypsum board, such as calcined gypsum, adhesive auxiliary agent, set accelerator, additives, admixtures and so forth; a slurry pouring step of pouring the produced gypsum slurry of the mixing step between upper and lower sheets of paper for gypsum board liner; a forming step of generally shaping the sheets and slurry so as to have a predetermined configuration of board; a severing and drying step of severing the continuous belt-like form of gypsum board into green boards and forcibly drying them; and a cutting step of finally cutting each of the dried boards to have a predetermined size of the product. In addition to such a widely used gypsum board, a lath board, decorative gypsum board, gypsum sheathing board, reinforced gypsum board and so forth are known in the art as board materials for building construction to be produced in accordance with similar methods. These board materials are defined in JIS (Japanese Industrial Standard; JIS A6901), as being various kinds of board materials to be selectable in correspondence with their purpose of use and performance, and they are actually placed on the market of building construction materials.
FIG. 12 is a schematic side view illustrating an arrangement of a conventional gypsum board manufacturing machine. In FIG. 12, there is shown a part of the machine, in which the mixing step, the slurry pouring step and the forming step are carried out.
The gypsum board manufacturing machine is provided with a mixer A which prepares the slurry by mixing the aforementioned ingredients for the gypsum board. A thin, pin-type mixer is used as the mixer A, in a lot of gypsum board manufacturing plants. In general, this kind of mixer comprises a flattened cylindrical housing which defines a mixing area (mixing chamber), and a rotary disc to be rotated within the housing. In a central area of an upper cover of the housing, there are located a plurality of inlet ports which introduces the materials to be mixed, such as calcined gypsum, mixing water and foam, into the housing. The housing is provided in its peripheral zone with a discharging port for discharging the mixture therethrough. The upper cover or upper plate is provided with a plurality of upper pins depending therefrom down to the proximity of the rotary disc. The rotary disc has a plurality of lower pins vertically fixed thereon and extending up to the proximity of the upper cover. The upper and lower pins are radially alternately arranged. A rotary shaft and a driving device for rotating the disc are connected with the disc. The components fed into the housing are stirred and mixed by rotation of the disc in operation of the driving device, and moved radially outward on the disc by the action of centrifugal force, and then, discharged onto a sheet of paper for gypsum board liner from a chute F located in a peripheral portion of the housing, as the gypsum slurry S1. This kind of mixer is disclosed in, for instance, U.S. Pat. Publication No. 3,459,620, Japanese Patent Laid-Open Publications Nos. 8-25342, 2000-262882 and 2000-6137, and so forth.
In the technical field of manufacture of gypsum boards, efforts of long years have been made to further reduce the weight of gypsum board while keeping or improving the quality thereof. For example, in the forcible drying step during manufacture of gypsum boards, the drying rate of the gypsum board is, in general, relatively quick at an edge part or edge zone (an edge portion), in comparison with its widthwise center part. Therefore, the edge portion is apt to cause lack of strength, dryout, defective bonding between the gypsum core and the gypsum board liner paper, and the like, owing to excessive drying. In order to prevent such a phenomenon, the density of slurry at the edge portions of the gypsum board is generally set to be higher than the density of slurry at the center part thereof.
For making the density of the side edge portions of gypsum board higher, an agitator for slurry (gypsum slurry agitator B) independent of the aforementioned mixer is normally used, as shown in FIG. 12. A part of gypsum slurry prepared by the mixer is fractionated through a slurry fractionation port E disposed on a peripheral outer wall of the mixer housing, and is introduced into the gypsum slurry agitator B rotating at a high speed. The agitator B causes the foam to be broken or disappear so that the gypsum slurry with high density is obtained, and deposits the high density gypsum slurry S2 on a zone of the gypsum board liner paper corresponding to the edge portion of gypsum board. This type of gypsum slurry agitator is called a hard edge mixer, and employment of such a hard edge mixer makes it possible to form a high density (high specific gravity) core at the edge parts of gypsum board without making the density (specific gravity) of the center part of gypsum board higher. This kind of gypsum slurry agitator is disclosed, e.g., in U.S. Pat. Publication No. 4,279,673.
The gypsum slurry of the mixer is also fractionated through slurry fractionation ports E′, E″ disposed on the peripheral outer wall of the mixer, and it is fed to gypsum slurry agitators C, D for roll coaters G, H. The agitators C, D agitates the gypsum slurry to discharge the high-density gypsum slurry S′, S″ onto the gypsum board liner paper. Each of the roll coaters forms a thin layer of high-density slurry on the surface of the sheet for improving the adhesiveness between the gypsum core and the paper.
Further, a mixer disclosed in Publication of PCT International Application No. WO 97-23337 has an arrangement in which inlets for feeding materials to be mixed, except foam, are disposed in a center area of the mixer. The mixer prepares gypsum slurry without foam in the mixer, and discharges it through a main discharge outlet as a core stream. A part of the slurry in the mixer is extracted as an edge stream, through an auxiliary slurry discharge outlet disposed on a peripheral outer wall of the mixer. Foam is introduced into the core stream of slurry in vicinity of the main discharge outlet, so that a difference in the density of slurry is given between the core stream and the edge stream.
As set forth above, the slurry with high density is fed to the parts of the sheet corresponding to the edge portions of the gypsum board. In the conventional technique, problems have been indicated wherein excessively high-density slurry is fed to the edge portion, owing to excessive agitation in the gypsum slurry agitator and the like. Such a high density slurry results in exfoliation of the core due to surface cracking, which is caused between a high density core portion and a low density core portion, and difficulty of in-situ nailing or screwing in vicinity of an edge of the gypsum board. As practical countermeasures against excessive high density of the slurry, the foam has been excessively added to the mixer in estimation of defoaming action of the slurry agitator, or the foam is added to the slurry in the slurry agitator through a foam inlet provided on the slurry agitator. However, such countermeasures are in contradiction to the intention of equipment of the gypsum slurry agitator (breaking the foam) for making the density of slurry higher. In addition, this results in undesirable increase of the consumption rate (the dosage of additive per a single standard gypsum board) of foam or foaming agent.
Further, in the conventional mixer, a fractionation port of the gypsum slurry is provided on a peripheral outer wall of the mixer, independent of a discharge port for depositing the gypsum slurry on the center part of the gypsum board liner paper. The gypsum slurry fractionated through the fractionation port (fractionated slurry) is apt to extensively vary in its density, compared to the gypsum slurry discharged from the chute. Thus, centralized control of the slurry density cannot be carried out, and control of the slurry density is, in practice, very difficult to be performed.
Furthermore, a mass of set slurry, which blocks the flow of slurry, tends to be produced in the mixer and a slurry delivery conduit (which is also called, fractionated slurry conduit or slurry fractionation conduit). This kind of slurry mass has a nature of growing as the operating time proceeds. Accordingly, the flow rate of slurry flowing through the conduit is reduced during operation, and thus, a problem of reduction of fractionated slurry arises.
Actually, the high density slurry discharged by the slurry agitator may exhibit its density significantly exceeding the predetermined target value or extremely less than the target value, owing to additional dosage of foam, scattering of the slurry density and change of the slurry flow rate. This results in a condition that distinct difference in density is not observed between the high-density slurry and the low-density slurry, or the difference is reversed. Thus, it is necessary to practice a reliable control in the density of fractionated slurry and restrict change in the slurry flow rate, in order to avoid loss of adhesiveness between the core and the gypsum board liner paper, lack of mechanical strength at the edge portion of the gypsum board, and the like (that is, deterioration of quality of finished product), and in order to prevent the foam consumption rate from increasing.
It is an object of the present invention to provide an apparatus and method for fractionating gypsum slurry, which can surely control the density of the gypsum slurry to be fractionated from the mixer, which can restrict the change in the flow rate of the fractionated slurry, and which can reduce the consumption of foam or foaming agent.
It is another object of the present invention to provide a method of producing gypsum boards, which enables stable production of high quality gypsum boards with use of such an apparatus for fractionating gypsum slurry.